@kingjs/define-generator NPM

@kingjs/define-generator

Define a generator given a MoveNext() factory; Poor man's yield.

Usage

Yield each argument passed to a function like this:

var defineGenerator = require('@kingjs/define-generator');

var sequence = defineGenerator(
  function moveNextFactory() {
    var array = arguments;
    var index = -1;
    return function moveNext() {
      if (index + 1 == array.length)
        return false;
      this.current_ = array[++index];
      return true;
    }
  }
);

Voila! Now, test sequence like this:

var enumerator = sequence(0, 1, 2);

var enumerator = enumerable.getEnumerator();

var actual = [];
while (enumerator.moveNext())
  actual.push(enumerator.current);
actual;

outputs:

[0, 1, 2];

API

declare function defineGenerator(
  moveNextFactory: (...args: any[]) => {
    moveNext: () => boolean
  }
): {
  getEnumerator: () => {
    moveNext: () => boolean,
    current: any
  }
}

Parameters

moveNextFactory: A function that returns a moveNext() function closed over the arguments passed to the generator.

Return Value

An enumerable that uses a moveNext() created by calling the moveNextFactory() with the arguments passed to the generator.

Remarks

With first class compiler yield support the above example could simply be:

function sequence() {
  for (var i = 0; i < arguments; i++)
    yield arguments[i];
}

While defineGenerator does not generate the state machine to track which element is next in line, hence the requirement to provide the moveNextFactory(), it does generate the type scaffolding, the Enumerable and Enumerator, which, given the state machine, returns a similar result.

Only "similar" because the official JavaScript yield returns an Iterator interface while defineGenerator returns an Enumerable interface ala C#.

One critique of the Iterator interface is that it creates a new object to wrap the result for every iteration and for termination. For example, an empty sequence requires creation of a { done: true } object when using Iterator whereas Enumerable simply returns false from moveNext().